Every year, agriculture globally suffers losses amounting to billions caused by plant pests, e.g. fungi or feeding insect pests, which attack and damage the leaves of useful plants. Until now these plant pests have been controlled with plant protectants, which according to the pesticide index (German Federal Office of Consumer Protection and Food Safety, as at 01.11.2007) belong to the areas:                herbicides against weeds,        insecticides against insect pests,        fungicides against fungal pathogens,        rodenticides against rodents,        nematicides against nematodes (threadworms),        acaricides against mites/arachnids,        molluskicides against snails,        bactericides against bacteria,        agents against viroids,        agents against viruses,        agents for vine grafting and grafting on fruit-bearing and ornamental shrubs,        agents for preventing damage by game,        agents for wound sealing/wound treatment,        growth regulators,        agents for treating seeds and planting stock, and        agents for soil disinfection        
All agents and substances have in common that they either stimulate the plant to be protected to suitable defensive measures or they kill the pests.
The plant leaf is, along with the shoot axis and the root, one of the three basic organs of higher plants and is known as the organ type phyllome. Leaves are lateral outgrowths on the nodes (nodi) of the shoot axis. The primary functions of the leaves are photosynthesis (synthesis of organic substances using light) and transpiration (water evaporation, important for nutrient uptake and transport). Leaves only occur with cormophytes, i.e. fern-like plants (Pteridophyta) and seed-bearing plants (Spermatophyta). Conversely, they are absent from mosses and algae, although leaf-like structures can form on their thallus, but these are only to be regarded as analogs of leaves. There is an enormous abundance of leaf shapes. In some cases, in the course of evolution leaf organs have also developed that no longer have anything to do with the original function of the leaf, namely photosynthesis and transpiration: for example petals, leaf thorns and leaf tendrils, and bud scales.
The leaf is sealed against the exterior with a boundary tissue, the epidermis, which consists of just one cell layer. The epidermis possesses on the outside a water-impermeable wax layer cuticle, which prevents unregulated evaporation. The cells of the epidermis do not as a rule possess any chloroplasts (the cell constituents in which photosynthesis takes place). Exceptions to this are the epidermis of hygro-, helo- and hydrophytes and sometimes shade leaves, but especially the guard cells of the stomata, which always contain chloroplasts. The stomata serve for regulation of gas exchange, primarily the release of water vapor. According to the distribution of the stomata, a distinction is made between hypostomatic leaves (stomata on the leaf undersurface, commonest form), amphistomatic leaves (stomata on both leaf surfaces) and epistomatic leaves (stomata on the leaf top surface, e.g. in the case of floating leaves). The appendages formed by the epidermis are called hairs (trichomes). If subepidermal cell layers are also involved in formation, these are called emergences: examples are spines or colleters. The assimilating tissue is called mesophyll. It is generally divided into the palisade parenchyma located under the upper epidermis and the spongy parenchyma located under that. The palisade parenchyma consists of one to three layers of oblong, chloroplast-rich cells standing perpendicularly to the leaf surface. In the palisade parenchyma, the main task of which is photosynthesis, there are about 80 percent of all chloroplasts. The spongy parenchyma consists of irregularly shaped cells, which owing to their shape form large intercellular spaces. The main task of the spongy parenchyma is to provide aeration of the parenchymal tissue. The cells are relatively poor in chloroplasts. The vascular bundles are often located on the boundary between palisade and spongy parenchyma in the upper spongy parenchyma. The structure is the same as that of the vascular bundles in the shoot axis and is generally collateral. The vascular bundles branch off from the shoot axis and pass through the leaf stalk without rotation into the leaf blade. As a result the xylem faces the upper surface of the leaf, and the phloem faces the leaf underside. Large vascular bundles are often surrounded by an endodermis, which is called bundle sheath here. The bundle sheath controls exchange of substances between vascular bundle and mesophyll. The vascular bundles end blind in the mesophyll. The vascular bundle is thereby reduced more and more, i.e. first the sieve tubes become fewer and disappear, then in the xylem part only spiral tracheids remain, and these finally end blind. The whole leaf is as a rule so densely traversed with vascular bundles that no leaf cell is more than seven cells away from a vascular bundle. The resultant small fields between the vascular bundles are called areolae or intercostal fields. The function of the vascular bundles is to transport water and minerals into the leaf (via the xylem) and to transport photosynthesis products away from the leaf (via the phloem).
So far no methods are known that produce, as protection against fungi and insect pests, a layer of whatever kind at all on the plant surface or leaf surface. It has been assumed until now that a coating would impair the physiology of the plant leaf and therefore would damage the plant. A coating as plant leaf-protecting layer must therefore fulfill two conditions. On the one hand sufficiently high translucence is required, in order to supply the chloroplasts contained in the plant leaf with radiation in the range from 320 to 700 nm. A coating that adsorbs or reflects in this wavelength range would impair the energy supply of the plant cell. The stroma is located as plasma phase in the interior of the chloroplasts. This stroma is traversed by thylakoid membranes (membrane invaginations), which stacked roll-like on top of one another form the granum. The chlorophyll embedded as pigment in the membranes can now once again adsorb light from the aforementioned wavelength range and utilize the absorbed energy for the production of ADP (adenosine triphosphate) from ADP (adenosine diphosphate) and phosphate.
The second requirement that a nanoscale plant leaf-protecting layer must fulfill is undisturbed function of the stomata. The gas exchange of a plant takes place through the stomata (Greek stoma, mouth). The stomata are normally formed by two bean-shaped cells, the guard cells, which surround an opening, the stoma. If we also include the cells that are located around the guard cells, we talk of the stomatal apparatus (stomatal complex). The pores themselves are strictly speaking the actual stomata. Guard cells are as a rule located in the lower epidermis of plant leaves, in the case of grasses on both sides of the leaf, and in the case of floating-leaf plants only on the upper surface. Gas exchange with the surrounding air is important in particular for supply of CO2. Carbon dioxide is absorbed by the plants from the air by the processes of photosynthesis. For optimum functioning of diffusion through the cell walls, these must be as thin and/or permeable as possible. However, such cells evaporate a lot of water, and with such leaves terrestrial plants would quickly wither. Through separation of the intercellular spaces in the leaf from the dry outside air by the stomata, the plant gains control over water loss. Other points are important for the stomata: evaporation (stomatal transpiration or evaporation) takes place through the pores, which produces suction, by which water is transported from the roots and into the leaves. With the water, nutrient salts are carried from the soil and are concentrated in the leaves. Additionally the evaporation cools the leaves, they do not overheat under strong insolation and the specific temperature optimum of the enzymes in the leaf tissues is not exceeded. The transpiration just over the area of the stomata, which only make up 1-2% of the total leaf surface area, is up to ⅔ of the evaporation, i.e. the resistanceless evaporation, of a water surface of equal area. Investigations have shown that many small openings at equal surface area evaporate more water. The reason is the so-called “edge effect”: molecules at the edge of a stoma can also diffuse sideways, whereas those in the middle hamper each other. The proportion of cuticular transpiration is very small, with hygrophytes (plants in moist areas) with tender leaves less than 10% of the evaporation of a free water surface, with trees less than 0.5% and with cacti even only 0.05%.
The stomatal apparatus consists of two guard cells, as a rule bean-shaped cells, which adhere to one another at both ends. Between them there is an intercellular gap, the pore, which forms the link between outside air and respiratory cavity. In some plants the two guard cells are surrounded by specialized epidermal cells, the subsidiary cells (pale blue in the illustrations), which are involved indirectly in opening and closing of the stoma. Leukoplasts can often be seen in the subsidiary cells. The guard cells contain chloroplasts, and so can carry out photosynthesis. The extent of opening of the pore is variable, in sunlight and with sufficient supply of water they are as a rule wide open, at night or with lack of water they are closed.